Plate tectonics theory describes how the Earth's outer shell is broken into rigid plates that constantly move across Earth's surface. There are three main types of plate boundaries - divergent where plates move apart, convergent where they move together, and transform where they slide past each other. As plates move, they interact along boundaries through processes like sea floor spreading at mid-ocean ridges, subduction at convergent zones, and earthquakes at transform faults. Over time, plate tectonics has shaped Earth through phenomena like mountain building, volcanism, and continental drift.

1. Chapter 4
The Way the Earth Works: Plate Tectonics
LECTURE OUTLINE
earth
Portrait of a Planet
Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

2. The Way the Earth Works:
Plate Tectonics
Prepared by
Ron Parker,
Earlham College Department of Geosciences
Richmond, Indiana

3. Plate Tectonics

The paradigm of “How the Earth Works.”
Earth’s outer shell is broken into rigid plates that move.
 Moving plates change the face of planet Earth.


A case study of a scientific revolution.

A powerful idea based on multiple lines of evidence.
It allows scientists to predict geologic events, and...
Reconstruct the geologic past.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

4. Plate Tectonics
Tectonic theory evolved in the 1960s.
 Previous research provided a strong foundation.

Wegener (1915) – Evidence supporting continental drift.
 Hess / Dietz (1960) – The sea-floor spreading hypothesis.

By 1968, evidence for tectonics was overwhelming.
 This evidence changed the view of most
geologists.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

5. Plate Tectonics
Plate tectonic theory is powerful.
 It provides a unified mechanism explaining:

Igneous, sedimentary, and metamorphic rocks.
 The distribution of earthquakes and volcanoes.
 The origin of continents and ocean basins.
 The distribution of fossil plants and animals.
 The genesis and destruction of mountain chains.
 Continental drift.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

6. Lithosphere

Tectonic plates are fragments of lithosphere.
Lithosphere is made of both crust and the upper mantle.
 The lithosphere is in motion over the asthenosphere.

Lithosphere bends elastically when loaded.
 Asthenosphere flows plastically when loaded.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

7. Buoyancy
First described by Archimedes more than 2.2 ka.
 Floating solids displace water equal to their mass.
 An iceberg “sinks” until the mass of water it
displaces is equal to the total mass of the iceberg.
 This concept applies to lithospheric plates.

Continental – Floats higher.
 Oceanic – Sinks lower.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

8. 2 Types of Lithosphere

Continental ~ 150 km thick.

Granitic crust.
35-40 km thick.
Lighter (less dense) .
More buoyant – Floats higher.

Oceanic ~ 7 to 100 km thick.

Basaltic crust.
7-10 km thick.
Heavier (more dense).
Less buoyant – Sinks lower.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

9. Plate Boundaries

Lithosphere is fragmented into ~ 20 tectonic plates.

Plates move continuously at a rate of 1 to 15 cm/yr.
This motion appears to be slow on a human time scale.
 Yet it is extremely rapid on a geologic time scale.


Plates interact along their boundaries.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

10. Plate Boundaries

Locations on Earth where tectonic plates meet.
Identified by concentrations of earthquakes.
 Associated with many other dynamic phenomena.


Plate interiors are almost earthquake-free.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

11. Continental Margins

Where land meets the ocean.
Margins near plate boundaries are “active.”
 Margins far from plate boundaries are “passive.”


Passive margin continental crust thins seaward.
Transitions into oceanic crust.
 Traps eroded sediment.
 Develops into the
continental shelf.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

12. Plate Boundaries: 3 Types

Divergent – Tectonic plates move apart.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

13. Plate Boundaries: 3 Types

Convergent – Tectonic plates move together.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

14. Plate Boundaries: 3 Types

Transform – Tectonic plates slide sideways.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

15. Divergent Boundaries
Sea-floor spreading causes plates to move apart.
 Magma wells up to fill the gap.
 Magma cools, adding material to each plate.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

16. Divergent Boundaries

Sea-floor spreading progression.

Early stage
Rifting has progressed to mid-ocean ridge formation.
Before substantial widening of the ocean.
Forms a long, thin ocean basin with young oceanic crust.

Example: The Red Sea
Note: This diagram only depicts the crust, not the entire lithosphere.
Time 1
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

17. Divergent Boundaries

Sea-floor spreading progression.

Mid-stage
Ocean begins to widen.
New seafloor is added at the mid-ocean ridge.
Continents move farther apart.

Example: Greenland and the North Atlantic
Note: This diagram only depicts the crust, not the entire lithosphere.
Time 2
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

18. Divergent Boundaries

Sea-floor spreading progression.

Late stage
Mature, wide ocean basin.
Linear increase in age with distance from central ridge.
Edge of ocean basin - oldest; ridge proximal - youngest.

Example: The Atlantic Ocean
Note: This diagram only depicts the crust, not the entire lithosphere.
Time 3
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

19. Mid-Ocean Ridges
Linear mountain ranges in Earth’s ocean basins.
 Example: The Mid-Atlantic Ridge

Snakes N-S through the entire Atlantic Ocean.
 Elevated ridge (1,500 km wide) 2 km above abyssal plains.
 Axial rift valley.

500 m deep.
10 km wide.
Symmetric.
Site of eruptions.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

20. Mid-Ocean Ridges
Sea-floor spreading opens the axial rift valley.
 Rising asthenosphere melts, forming mafic magma.


Pooled magma solidifies into oceanic crustal rock.
 Pillow
basalt – Magma quenched at the sea floor.
 Dikes – Preserved magma conduits.
 Gabbro – Deeper magma.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

21. Mid-Ocean Ridges

“Black smokers” are found at some MORs.
Water entering fractured rock is heated by magma.
 Hot water dissolves minerals and cycles back out of rock.
 When water reaches the sea, minerals precipitate quickly.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

22. Ocean Crustal Age

Oceanic crust spreads away from the ridge axis.
New crust is closer to the ridge; older crust farther away.
 Oldest oceanic crust is found at the far edge of the basin.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

23. Oceanic Lithosphere
The hot asthenosphere is at the base of the MOR.
 Aging ocean crust moves away from this heat…

 Cooling,
increasing in density, and sinking.
 Accumulating increasing thicknesses of sediment.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

24. Convergent Boundaries
Lithospheric plates move toward one another.
 One plate dives back into the mantle (subduction).
 Subduction recycles oceanic lithosphere.

Subduction is balanced by sea-floor spreading.
 Earth maintains a constant circumference.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

25. Subduction
Old oceanic lithosphere is more dense than mantle.
 A flat-lying oceanic plate won’t subduct.
 Bent down, the leading edge sinks like an anchor.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

26. Convergent Boundaries

The subducting plate descends at an average of 45° .

Plate descent is revealed by Wadati-Benioff earthquakes.
Mark frictional contact and mineral transformations.
Earthquakes deepen away from trench.

Quakes cease below 660 km.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

27. Fate of Subducted Plates?
Plate descent continues past the earthquake limit.
 The lower mantle may be a “plate graveyard.”

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

28. Subduction Features

Subduction is associated with unique features.
Accretionary prisms.
 Volcanic arcs.
 Back-arc basins.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

29. Convergent Boundaries

Accretionary prisms – Deformed sediment wedges.
Sediments scraped off subducting plates are smeared
and welded onto the overriding plates.
 These contorted sediments can be pushed above sea.

Washington’s Olympic Peninsula.
Taiwan.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

30. Convergent Boundaries

Volcanic arc – Volcanic belt on an overriding plate.
The descending plate partially melts at ~ 150 km depth.
 Magmas burn upward, fueling volcanic eruptions.
 A curved Earth dictates that volcanic belts are curved.


Arc type depends on overriding plate.
Continental crust – Continental arc.
 Oceanic – Island arc.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

31. Convergent Boundaries

Back arc basin – A marginal sea behind an arc.
Forms between an island arc and a continent.
 Offshore subduction traps a piece of oceanic crust, or…
 Stretching creates a new spreading ridge.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

32. Transform Boundaries

Lithosphere slides past; not created or destroyed.
Many transforms offset spreading ridge segments.
 Some transforms cut through continental crust.


Characterized by…
Earthquakes.
 Absence of volcanism.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

33. Oceanic Transforms

The MOR axis is offset by transform faults.
Offset of linear MOR is geometric necessity on a sphere.
 Transforms bear strong evidence of sea-floor spreading.

Abundant earthquakes common between ridge segments.
Earthquakes vanish past ridge segment overlaps.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

34. Transform Boundaries

Oceanic transforms – Offsets along the MOR.

Older interpretation – Faulting occurs after MOR forms.

Modern interpretation – Faulting occurs with the MOR.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

35. Transform Boundaries

Continental transforms – Chop continental crust.

Example: The San Andreas Fault.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

36. Triple Junctions
Places where 3 plate boundaries coincide.
 Multiple boundary combinations occur.
 Triple junctions migrate and change across time.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

37. Hot Spots

Volcanic plumes independent of tectonic plates.
Mafic magmas derived from the lower mantle.
 Tattoo overriding plates with volcanoes.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

38. Hot Spots
Volcanoes perforate overriding plates.
 Make volcanoes that drift off-plume.

Volcano goes extinct and erodes.
 Subsidence creates a guyot.
 Supports sea-floor spreading.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

39. Hot Spots

Hot spot volcanoes create seamounts.
Seamounts age away from originating hot spot.
 Age change marks direction of overriding plate motion.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

40. Tectonic Boundaries Evolve
Plate boundaries change over geologic time.
 Oceanic plates.

Created at MOR spreading centers.
 Destroyed at subduction zones.


Continental plates.
Torn apart at rifts.
 Joined during collision.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

41. Continental Rifting

Continental lithosphere can break apart.
Lithosphere stretches and thins.
 Brittle upper crust faults.
 Ductile lower crust flows.
 Asthenosphere melts.
 Melts erupt, leading to…


Sea-floor spreading.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

42. Continental Rifting

Example: East Africa.
The Arabian plate is rifting from the African plate.
 Rifting has progressed to sea-floor spreading in…

The Red Sea
The Gulf of Aden

East African Rift – On-going rift.
Thinned crust.
Elongate trough (channel).
Volcanoes.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

43. Plate Collision
Subduction consumes ocean basins.
 Ocean closure ends in continental collision.

 Buoyant
continental crust will not subduct.
 Subduction ends; mountains are smashed upward.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

44. Plate Collision

Plate tectonic collision may involve…
Two continents.
 A continent and an island arc.


Collision “sutures” the convergent plate boundary.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

45. Driving Mechanisms

Old: Plates are dragged atop a convecting mantle.
Plate motions are much too complex.
 Convection does occur.

It is not the prime
driving mechanism.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

46. Driving Mechanisms

Modern: 2 other forces drive plate motions.
Ridge-push – Elevated MOR pushes adjacent lithosphere.
 Slab-pull – Gravity pulls a subducting plate downward.
 Convection in the asthenosphere adds or subtracts.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

47. Plate Velocities

Absolute plate velocities may be mapped by…
Plotting plate motion relative to a fixed spot in the mantle.
 Measuring volcano ages / distance along a hot spot track.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

48. Plate Velocities

Plate vectors are determined GPS measurements.
Global Positioning System (GPS) uses satellites.
 Knowledge of plate motion is now accurate and precise.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

49. The Dynamic Planet

Plate tectonics: The key to understanding geology.
Mantle is transferred to the surface and back down again.
 The interior and surface of Earth are in constant motion.
 Explains earthquakes, volcanoes, and continental drift.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

50. The Dynamic Planet

Earth’s surface changes continuously.
These changes appear slow to us.
 Geologically, change is rapid.

Earth looked different in the past.
 Earth will look different in the future.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

51. The Dynamic Planet

Plate Tectonics Summary: Ocean floor created at mid-ocean
ridges is consumed at oceanic trenches.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

52. The Dynamic Planet

Plate Tectonics Summary: Ocean floor created at mid-ocean
ridges is consumed at oceanic trenches.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics

53. This concludes the
Chapter 4
The Way the Earth Works: Plate Tectonics
LECTURE OUTLINE
earth
Portrait of a Planet
Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 4: The Way the Earth Works: Plate Tectonics